Display devices, such as a cellular phone, a digital camera, a PDA, a touch panel display, and a large-screen television, show a tendency of further prevalence.
Hitherto, in those applications, a resin sheet, such as an acrylic sheet, has been used as a protective member for protecting a display. However, owing to a low Young's modulus of the resin sheet, the resin sheet is liable to bend when a display surface of the display is pushed with a pen, a human finger, or the like. Therefore, the resin sheet causes a display failure through its contact with an internal display in some cases. The resin sheet also involves a problem of being liable to have flaws on its surfaces, resulting in easy reduction of visibility. A solution to those problems is to use a glass sheet as the protective member. The glass sheet for this application is required to, for example, (1) have a high mechanical strength, (2) have a low density and a light weight, (3) be able to be supplied at low cost in a large amount, (4) be excellent in bubble quality, (5) have a high light transmittance in a visible region, and (6) have a high Young's modulus so as not to bend easily when its surface is pushed with a pen, a finger, or the like. In particular, a glass sheet which does not satisfy the requirement (1) cannot serve as the protective member, and hence a tempered glass sheet obtained through ion exchange treatment has been used as the protective member heretofore (see Patent Literatures 1 and 2, and Non Patent Literature 1).
Hitherto, the tempered glass sheet has been produced by so-called “pre-tempering cutting”, which is a method comprising cutting a glass sheet to be tempered so as to have a predetermined shape in advance and then subjecting the resultant to ion exchange treatment. In recent years, so-called “post-tempering cutting”, which is a method comprising subjecting a large glass sheet to be tempered to ion exchange treatment and then cutting the resultant so as to have a predetermined size, has been under consideration. When the post-tempering cutting is performed, there is an advantage in that the manufacturing efficiency of the tempered glass sheet and various devices dramatically improves.